FIGS. 2 and 3 illustrate a chain transmission device for transmitting rotation between a crankshaft and a camshaft of an engine with a ratchet tensioner which has been used widely to apply a proper tension onto a free run of a timing belt or a timing chain for dampening vibrations while the belt or chain is running.
The ratchet tensioner A1 is mounted on a body of the engine at the slack side of the chain (timing chain) A6 extending between a drive sprocket A3 rotatably driven by a crankshaft A2 of the engine and a driven sprocket A5 firmly secured to a camshaft A4. The ratchet tensioner A1 includes a plunger A8 partly projecting from the from the front surface of a tensioner housing A7, and a tensioner lever A10 pivotally supported on the engine body via a support shaft A9. A free end F of the plunger A8 engages the back side of the tensioner lever A10 and biases the free end portion so that a shoe surface A11 on the front side of the tensioner lever A10 is urged onto sliding contact with a free run of the chain A6 to thereby apply a tension onto the chain A6. The tensioner housing A7 has a plunger receiving hole A12 in which the plunger A8 is slidably received for axial displacement relative to the tensioner housing A7. Adjacent the free end F, the plunger has a free end portion projecting from the hole A12.
The plunger A8 has an internal space or hollow portion A13 open at one end face of the plunger A8 which is disposed near the bottom the plunger receiving hole A12. A plunger spring A14 fits in the hollow portion A13 of the plunger A8 and acts between the bottom of the plunger receiving hole A12 and the bottom of the hollow portion A13 and normally urges the end portion of the plunger A8 to project from the plunger receiving hole A12 of the tensioner housing A7.
A ratchet A16 is pivotally supported on the tensioner housing A7 via a ratchet shaft A15. As shown in FIG. 3, the ratchet A16 has a first ratchet prong A17 and a second ratchet prong A18 both engageable with ratchet teeth T formed on one side of the plunger A8. By a ratchet spring A19 disposed between the ratchet A16 and the tensioner housing A7, the ratchet A16 is normally urged to turn about the ratchet shaft A15 in such a direction that the first and second ratchet prongs A17, A18 are held in mesh with the ratchet teeth T on the plunger A8. The prongs are spaced apart by an integral multiple of the tooth spacing in the illustrated device, the prong spacing is twice the tooth spacing. Mainly by meshing engagement between the first ratchet prong A17 and the ratchet teeth T, the plunger A8 is prevented from displacing in the backward direction. The second ratchet prong A18 is provided to increase the amount of backlash when the first ratchet prong A17, which is in mesh with one ratchet tooth T, comes in meshing engagement with the next ratchet tooth T as the plunger A8 is displaced forwardly. Tensioners having such ratchet with double ratchet prongs are described in Japanese Utility Model Registration No. 2516271 Japanese Patent Publication No. HET 3-8415.
In the ratchet tensioner A1 shown in FIG. 1, the plunger receiving hole A12 of the tensioner housing A7 and the hollow portion A13 of the plunger A8 jointly form a high pressure chamber H into which an oil is supplied from the outside of the tensioner housing A7 through an oil passage A20 and a ball check valve A21 so as to dampen or absorb pulsation of the tension tending to apply an impact or shock force onto the chain A6 while the chain A6 is running. When an impact or shock force is exerted from the chain A6 through the tensioner lever A10 against the free end F of the plunger A8, the plunger A8 retracts within a range of the backlash. In this instance, the oil in tho high pressure chamber H is compressed to develop a rapid pressure rise in the high pressure chamber H whereupon a check ball A22 of the ball check valve A21 is forced against a ball seal A23 to thereby close an opening which is in fluid communication with the oil passage A20. Consequently, the oil is trapped in the high pressure chamber H, and as the pressure inside the high pressure chamber H further increases, the oil is forced to leak through a slight clearance between the outer peripheral surface of the plunger A8 and the inner peripheral surface of the plunger receiving hole A12 to the outside of the plunger housing A7. During that time, the oil produces a flow resistance which is effective to dampen or absorb the impact or shock force acting on the plunger A8.
Prolonged use of the tensioner causes an elongation of the chain A6 which may permit the plunger A8 to be displaced in the forward or projecting direction beyond the backlash whereupon the first and second ratchet prongs A17 and A18 are brought into meshing engagement with other teeth T than as engaged before. Thus, the tensioner lever A10 while being biased by the plunger A8 can automatically take up or accommodate the elongation of the chain A6.
In the ratchet tensioner of the foregoing construction, as shown in FIG. 3, the second ratchet prong A18 is designed to mesh with the ratchet tooth T at a position located on an imaginary line N perpendicular to the axial line of travel of the plunger and passing through the axis O of the ratchet shaft A15, when the first ratchet prong A17 is in mesh with the ratchet tooth T. The first and second ratchet prongs A17 and A18 are manufactured such that an extent of projection (projecting length) S1 of the first ratchet prong A17 measured from the axis O of the ratchet shaft A15 to a top of the prong A7 in a direction parallel to the perpendicular line N and an extent of projection (projecting length) S2 of the second ratchet prong A18 measured from the axis O of the ratchet shaft A15 to a top of the prong A18 in a direction parallel to the perpendicular N, considering the manufacturing tolerances of the respective projection extents are substantially the same. Even though both prongs A17 and A18 are manufactured within allowable tolerance, the projecting length S2 of the prong 18 may be greater than the projecting length S1 of the prong A17 in which instance the second ratchet prong A18 tends to interfere with a bottom surface or land between two adjacent ratchet teeth T to thereby lock the plunger A8 in position. Once the plunger A8 is locked in position, it is no longer possible to make the tensioner lever A10 adaptable to wear elongation of the chain A6, thus failing to permit the prescribed function of the tensioner. To avoid the foregoing problem, an attempt was made to increase the accuracy to such an extent to make the projecting length S1 of the first ratchet prong A17 precisely equal to the projecting length S2 of the second ratchet prong A18. However, the attempted process requires higher part accuracies and hence lowers the productivity in the manufacturing of the parts.
Further, even if the projecting length S1 of the first ratchet prong A17 and the projecting length S2 of the second ratchet prong A18 are made equal each other, there still arises a problem that when fine foreign matter or contaminant, such as metal particles produced due to abrasive wear from sliding portions within the engine enters together with a lubricant the slight clearance between the plunger receiving hole A12 of the tensioner housing A7 and a portion of the outer peripheral surface of the plunger A8 opposed from a peripheral surface portion on which the ratchet teeth T are formed. In such a case, the particles may cause lateral displacement or tilting of the plunger A8 and displacement of the plunger teeth toward the ratchet shaft A15. As a result, the second ratchet prong A18 interferes with a bottom land two adjacent ratchet teeth T to thereby lock the plunger A8 in position.